Helical stent

ABSTRACT

An expandable balloon for insertion in a fluid conduit of a human or animal body is movable between a collapsed condition and an expanded condition. The balloon has, when in the expanded condition, a center line which follows a substantially helical path. The balloon can be combined with a stent such that the stent is expandable by the balloon from a collapsed condition to an expanded condition.

This application is a continuation of U.S. application Ser. No.10/549,355 which was filed on Jul. 31, 2006 and is still pending. Thatapplication, in turn, was the national phase of InternationalApplication No. PCT/GB2004/001155 which was filed Mar. 18, 2004 andwhich claims priority from British Application Serial No. 0306176.9filed Mar. 18, 2003 and British Application Serial No. 0317003.2 filedJul. 21, 2003.

This invention relates to stents for insertion in a fluid conduit of thehuman or animal body.

Stents are generally tubular devices used for providing physical supportto blood vessels, i.e. they can be used to help prevent kinking orocclusion of blood vessels such as veins or arteries and to preventtheir collapse after dilatation or other treatment.

Stents can be broadly divided into two main categories: balloonexpandable stents and self-expanding stents. In the case of the formerthe material of the stent is plastically deformed through the inflationof a balloon, so that after the balloon is deflated the stent remains inthe expanded shape. Such stents are manufactured in the “collapsed”condition, ready for delivery, and may be expanded to the expandedcondition when inside the vessel or other fluid conduit.

Self-expanding stents are also designed to be delivered in the collapsedcondition and when released from a constraining delivery system thestent expands to its expanded condition of a predetermined size. Thiseffect is achieved by using the elasticity of the material and/or ashape-memory effect. In the case of shape-memory stents a commonly usedmaterial is nitinol.

Many different designs of stents are available on the market. They aremade from a variety of materials providing corrosion resistance andbiocompatibility. They are made from sheet, round or flat wire ortubing. They are generally cylindrical but also longitudinally flexibleso as to conform to the curvature of the fluid conduit into which theyare inserted.

It has been proposed in EP 1042997 to provide stents the flexibility ofwhich varies along their length, in order to facilitate placement of oneend of the stent in a narrower or tortuous coronary artery, or toachieve stenting of a bend of a particular coronary artery. Thisproposal involves providing the stent with a pattern of interconnectedstruts, with the strut thickness being variable along the length of thestent.

We have previously proposed that the flow pattern in arteries includingthe swirling pattern induced by their non-planar geometry operates toinhibit the development of vascular diseases such as thrombosis,atherosclerosis and intimal hyperplasia.

In WO 98/53764, there is disclosed a stent for supporting part of ablood vessel. The stent includes a supporting portion around which orwithin which part of a blood vessel intended for grafting can be placedso that the stent internally or externally supports that part. Thesupporting portion of the stent is shaped so that flow between graft andhost vessel is caused to follow a non-planar curve. This generates aswirl flow, to provide a favourable blood flow velocity pattern whichreduces the occurrence of vascular disease, particularly intimalhyperplasia.

In WO 00/32241, there is disclosed another type of stent, in this caseincluding a supporting portion around which or within which part of anintact blood vessel other than a graft can be placed. This supportingportion can prevent failure of the vessel through blockage, kinking orcollapse. Again, the supporting portion of the stent is of a shapeand/or orientation whereby flow within the vessel is caused to follow anon-planar curve. Favourable blood flow velocity patterns can beachieved through generation therein of swirl flow within and beyond thestent. Failures in blood vessels through diseases such as thrombosis,atherosclerosis, intimal hyperplasia can thereby be significantlyreduced.

Further aspects of how swirl flow is beneficial are explained in theabove publications. It is further explained in Caro et al. (1998) J.Physiol. 513P, 2P how non-planar geometry of tubing inhibits flowinstability.

It has been proposed in WO 00/38591 to provide a stent with internalhelical grooving or ridging to induce helical flow. FIGS. 9 to 12 ofthis document show a stent in the form of a mesh cylinder, with vanemembers attached to the inside of the cylinder so as to project into thefluid passage and guide the flow. However the presence of vanesprojecting into the flow may obstruct the flow and increase flowresistance, especially if there is any build-up of material on thevanes. Also, the use of vanes in an otherwise cylindrical tube may notreliably induce swirl flow across the entire cross-section of flow.There may be a tendency for the flow nearer to the centre of the tube tofollow a linear path, particularly for flows at higher Reynolds numbers.Further, the provision of vanes over a relatively short length of flowis likely to create only a temporary alteration of flow characteristics,with the flow reverting to a normal pattern at a distance downstream ofthe vanes.

In WO 02/098325 there are various proposals for cylindrical externalstructures for placement outside of blood flow conduits in order toinfluence the internal geometry of the conduit lumen. By providing ribsor other radially inwardly projecting helical members, thecross-sectional shape of the lumen is modified from the outside of theconduit. The various structures are not for use as stents capable ofdelivery internally of a conduit in a collapsed condition and forexpansion at the target site.

In WO 00/32241 internal stents for establishing and/or maintainingnon-planar curvature of a blood vessel are shown. FIG. 5 of thisdocument shows a clip which is part coiled or at least part helical ofshape memory alloy, affixed to a cylindrical wire mesh. With such anarrangement, when the clip moves to a more coiled condition once thestent has been installed, it will cause the cylindrical wire mesh toadopt a non-planar curvature but it will also cause it to twist. Sinceit is undesirable for the stent to apply torsional loading to the insidewall of the blood vessel, this twisting effect may limit the number ofhelical turns imposed by the clip, for example to one or less than oneturn. However, the objective of inducing or maintaining swirl flow inthe vessel is assisted by increasing the number of helical turns. Theclip also forms a rib projecting into the flow lumen of the blood vesselwhich, as discussed above in relation to vane members, may not be idealfor the flow characteristics of the vessel.

We have now found a way of producing an internal stent capable of movingfrom a collapsed condition to an expanded condition without significanttwisting but which facilitates flow within the stent supported fluidconduit to follow a non-planar curve, i.e. to swirl.

According to a first aspect of the invention there is provided a stentfor insertion in a fluid conduit of the human or animal body when thestent is in a collapsed condition and for expansion to an expandedcondition, the stent comprising an outer wall for engagement with theconduit, the outer wall having a helical portion which in the expandedcondition extends longitudinally and circumferentially, and which, uponexpansion of the stent from the collapsed condition to the expandedcondition resists extension.

Flow within the fluid conduit supported by such a stent can follow anon-planar curve, promoting swirl flow, the benefits of which arediscussed above. Thus, considering the flow lumen of the conduit, asthis extends in the longitudinal direction (x-axis) it curves in morethan one plane (i.e. in both the y-axis and the z-axis). In other words,the flow lumen extends generally helically in the longitudinaldirection. Such a non-planar curve may be achieved by a non-rotationallysymmetric shape (rotational symmetry of order one) when in the expandedcondition, which “twists” along the length of the stent. It may howeveralso be achieved if the stent has a circular or other rotationallysymmetrical cross-sectional shape, providing the cross-section as awhole shifts laterally from one “slice” to the next. In some instances,a combination of a non-rotationally symmetric shape which twists andwhich shifts laterally is provided.

Preferably, when the stent is in its expanded condition, it causes thefluid conduit to follow a non-planar curve as it extends in thelongitudinal direction and the curve undergoes at least one turn. Theprovision of more than one turn, more preferably a plurality of turns,is enabled by the ability of preferred stents to expand from thecollapsed condition to the expanded condition without substantialtwisting, i.e. there is no significant rotation of one end of the stentrelative to the other. This can be achieved by the helical portionhaving the same number of turns both when the stent is collapsed andwhen it is expanded. This property of the stent means that it can expandwithout causing the conduit to twist, which would be undesirable becauseof the tethering of the conduit in the human or animal body.

Preferably, the centre line of the stent in the expanded conditionfollows a substantially helical path. In other words, the centroids ofadjacent cross-sectional slices through the stent define a helical locusor centre line.

It is generally preferred to avoid any pronounced grooves, ridges, ribsor vanes, as these may have the opposite of the desired effect ofimproving flow characteristics, i.e. they may obstruct the flow,facilitate deposit build up or create stagnant regions. Preferably,therefore, the stent is substantially free of ribs or vanes, for examplefree of thicker wires (than adjacent wires) which would act as a ribprojecting into the flow lumen of the flexible conduit.

The stent improves flow characteristics. As is well known, in the caseof straight tubes, near wall velocities are very low compared tovelocities at the core of the tube, due to the effects of viscosity. Inthe case of tubes which are bent in a single plane, the speed of theflow at the outside of the bend is increased but the speed of the flowat the inside is retarded further. In both cases, there is considerablevariation in axial velocity across the width of the tube. With the useof a helical tubing portion according to the invention, a swirl flow isgenerated and the axial velocity profile of the flow across the tubingportion becomes generally more uniform or “blunter”, with the axialvelocity of flow at both the outside and inside of the tubing portionbeing closer to the mean axial velocity.

Thus, the flow characteristics are improved by causing swirling and arelatively uniform distribution of axial and near wall velocity. Mixingover the cross section is also promoted and there is a reduction in thelikelihood of occurrence of flow instability. The avoidance and flushingof stagnant zones is assisted. There is a reduction in the potential fordeposit build up within and downstream of the graft and the developmentof pathology.

The amplitude and pitch of the helical centre line may be chosen to varyalong the length of the stent, if desired. Variation of amplitude can beachieved by increasing or decreasing the resistance to extensionprovided by the helical portion, whilst variation in pitch may beachieved by varying the pitch of the helical portion itself. Suchvariations may for example be desired if it is wished to introduce agentle swirl at the upstream end of the stent and to increase the swirleffect in the downstream direction.

The stents of the preferred embodiments have a helical portion which hasa greater resistance to extension than portions of the stent adjacent tothe helical portion. Preferably, the helical portion comprises anincreased amount of stent forming material relative to the amount ofstent forming material in portions of the stent adjacent to the helicalportion. The increased amount of material can provide the requiredresistance to extension when the stent expands to the expandedcondition. The increase may be provided for example by thickerstructural members, in the radial direction and/or longitudinaldirection and/or circumferential direction. The increased amount mayalternatively or additionally be provided by the use of extra stentforming members. For example, in the case of a woven stent, the helicalportion may be provided by weaving in one or more extra wires. In othercases, extra struts may be provided.

The helical portion may comprise structural members having bent portionswhich resist unbending during expansion of the stent. Many stentsconsist of structural members bent between nodes or at nodes. Ingeneral, when the stent expands some or all of the bent portions unbendas the diameter of the stent increases. The desired resistance toextension may therefore be achieved by the helical portion havingstructural members with bent portions which resist unbending more thanbent portions adjacent to the helical portion.

The helical portion may be arranged to resist extension in thecircumferential direction, or to resist extension in the longitudinaldirection, or to resist extension in the circumferential and thelongitudinal directions. The choice of the appropriate form of thehelical portion will generally depend on the type of stent and themanner in which it expands.

The helical portion may be viewed as a helical stripe extendinglongitudinally and circumferentially of the stent. The stripe may besubstantially continuous, as for example in the case of one or moreextra wires woven into the stent, or it may be discontinuous, as will bethe case where the stent has thicker or otherwise modified structuralmembers which are separated by spaces.

The stent may be of the self-expanding type or it may be balloonexpandable. In the case of self-expanding stents, during expansion fromthe collapsed condition to the expanded condition, the portions whichare not part of the helical portion will be seeking to expand due totheir elasticity or shape-memory properties. The expansion is resistedin the vicinity of the helical portion by being less expansible. Thehelical portion may itself extend to some degree during expansion of thestent, and indeed may itself be seeking to expand due to its elasticityor shape-memory properties. However, the rest of the stent will beseeking to expand more than the helical portion so that in effect thehelical portion provides a resistance to extension. This will enable thestent to assume the desired shape for promoting swirl flow in a fluidconduit supported by the stent.

In the case of a balloon expandable stent, the force to expand the stentis supplied by the balloon and the helical portion will allow lessexpansion, which will normally mean a lesser degree of plasticdeformation, than the rest of the stent.

The basic geometry of the stent may be of the many available types, suchas coil stents, helical spiral stents, woven stents, sequential ringstents, closed cell sequential ring stents, and open cell stents. Theymay be made by coiling, braiding or knitting wires, by laser cuttingfrom tubing, by electric discharge milling (EDM), by chemical etching orby other known methods. They may be made from a variety of materials,including stainless steel, nitinol, tantalum, platinum iridium, niobiumalloy, cobalt alloy or polymers (such as biodegradable polymers).

According to a second aspect of the invention there is provided aballoon expandable stent for insertion in a fluid conduit of the humanor animal body when the stent is in a collapsed condition and forexpansion to an expanded condition, the stent comprising a balloonhaving an expandable wall, the wall having a helical portion which inthe expanded condition extends longitudinally and circumferentially, andwhich, upon expansion of the balloon from the collapsed condition to theexpanded condition, resists extension.

In some circumstances the main stent body, i.e. that which is left inthe fluid conduit after the balloon is removed, may be of a conventionaltype before expansion. After expansion, however, it retains (by plasticdeformation) a shape which corresponds to that determined by the balloonwith the helical portion of reduced extensibility.

Alternatively, the stent may have an outer wall for engagement with thefluid conduit in accordance with the first aspect of the invention, i.e.also with a helical portion which resists extension. The helicalportions of the balloon and the stent outer wall would then preferablybe arranged in registration with each other.

The helical portion of the balloon expandable wall may have a wallthickness greater than that of adjacent wall portions. This could easilybe achieved by adding a helical “stripe” around the outside of a balloonof uniform wall thickness, thereby creating the thicker helical portion.

In certain aspects, the invention is concerned with stents for insertionin flexible conduits of the human or animal body, in which a helicalcentre line of the flow lumen of the conduit is of relatively smallamplitude.

A further proposal in WO 00/38591 is to provide a circular-section tubebent into a cork screw shape. It is usual for the helix of a cork screwto have a clear gap down the middle, so that this proposed configurationwould have a wide swept width compared to the width of the tubing,certainly more than two tubing diameters. The amplitude of the helixwould be greater than one half of the internal diameter of the tubingand there would be no “line of sight” along the inside of the tubing.This proposal would therefore be relatively bulky and unsuitable forcertain applications. A similar proposal is shown in FIG. 5 of WO02/98325, relating to a tubular mesh structure to be located externallyof a conduit, the tubing having a helix with a large amplitude and againno “line of sight” along the inside of the tubing.

According to a third aspect of the invention, there is provided a stentfor insertion in a fluid conduit of the human or animal body when thestent is in a collapsed condition and for expansion to an expandedcondition, wherein in the expanded condition the stent causes the fluidconduit to have a flow lumen having a centre line which follows asubstantially helical path, the helical centre line having a helix angleless than or equal to 65° and an amplitude less than or equal to onehalf of the internal diameter of the flow lumen.

The features of the third aspect above, and its preferred featuresbelow, may also be useful in conjunction with the stents in accordancewith the first or second aspects of the invention, individually or incombinations.

The invention is applicable to stents internal to intact blood vesselsor blood vessels intended for grafting.

In this specification, the amplitude of the helix refers to the extentof displacement from a mean position to a lateral extreme. So, in thecase of the flow lumen having a helical centre line, the amplitude isone half of the full lateral width of the helical centre line.

In the flow lumen, in which the amplitude of the helix is less than orequal to one half of the internal diameter of the tubing, there is a“line of sight” along the lumen of the tubing, unlike in the case of acorkscrew configuration where in effect the helix is wound around a core(either solid, or “virtual” with a core of air). We have found that theflow at the line of sight generally has a swirl component, even thoughit could potentially follow a straight path.

For the purposes of this specification, the term “relative amplitude” ofa helical flow lumen is regarded as the amplitude divided by theinternal diameter. So, in the flow lumen in which the amplitude of thehelical tubing is less than or equal to one half of the internaldiameter of the tubing, this means that the relative amplitude is lessthan or equal to 0.5. Relative amplitudes less than or equal to 0.45,0.4, 0.35, 0.3, 0.25, 0.2, 0.15 or 0.1 may be preferred in somecircumstances. It is however preferred for the relative amplitude to beat least 0.05, more preferably 0.1. This can help to ensure that thedesired swirl flow is induced.

The relative amplitude may vary according to the use of the stent andthe spatial constraints on its design. It will however be appreciatedthat by keeping the amplitude less than half the tubing internaldiameter a swirling flow may be induced without creating an excessivelylarge device. The “envelope” occupied by the stented conduit can fitinto the space available in the tissue surrounding the fluid conduit,and even if this envelope is caused to follow a particular path by thelocal environment in which the conduit is located, the desired helicalgeometry of the flow lumen can be maintained.

It is expected that the conduit may prevent the stent from expanding toits full size. Therefore, the stent may be designed to have a relativeamplitude greater than 0.5 (e.g. 0.6 or 0.7), but so that in use arelative amplitude of the flow lumen is equal to or less than 0.5. Incertain preferred arrangements, however, the relative amplitude of theexpanded stent ex vivo is less than or equal to 0.5.

The angle of the helix is also a relevant factor in balancing the spaceconstraints on the flow tubing with the desirability of maximising thecross-sectional area available for flow. The helix angle is less than orequal to 65°, preferably less than or equal to 55°, 45°, 35°, 25°, 20°,15°, 10° or 5°. As with relative amplitudes, the helix angle may beoptimized according to the conditions: viscosity, density and velocityof fluid.

Generally speaking, for higher Reynolds numbers the helix angle may besmaller whilst satisfactory swirl flow is achieved, whilst with lowerReynolds numbers a higher helix angle will be required to producesatisfactory swirl. The use of higher helix angles will generally beundesirable, as there may be near wall pockets of stagnant fluid.Therefore, for a given Reynolds number (or range of Reynolds numbers),the helix angle will preferably be chosen to be as low as possible toproduce satisfactory swirl. Lower helix angles result in smallerincreases in length as compared to that of the equivalent cylindricaltubing. In certain embodiments, the helix angle is less than 20° or lessthan 15°.

It will be appreciated that in pulsatile flow, the Reynolds number willvary over a range. Typical mean resting arterial blood flow Reynoldsnumbers are about 100, reaching peak values of two or three times thatin pulsatile flow and three to four times the mean during exertion.Therefore the extent to which swirl flow is promoted will vary likewise.Even if there are stagnant flow regions at lower Reynolds numbers,because for example a low helix angle and/or a low relative amplitudehas been selected, these will tend to be flushed out during periods offlow when the Reynolds numbers are higher.

The stent may be made with substantially the same relative amplitude andhelix angle along its length. There may be small variations when thestent is in use, caused by elongation or contraction of the tubingportion due to tensile loading or caused by torsional loading. However,there may be circumstances in which the stent has a variable helix angleand/or relative amplitude, either to suit the space constraints or tooptimise the flow conditions.

For reasons of manufacturing simplicity, it may be preferred for thestent to have a substantially constant cross-sectional area along itslength. Again, there may be variations in use caused by loading on thestent.

The helical part of the stent may extend along just part of the overalllength of the stent or it may extend over substantially its entirelength. For example, a stent may have a part with the geometry of theinvention over part of its length or over substantially its entirelength.

The stent may undergo a fraction of one complete turn, for example onequarter, one half or three quarters of a turn. Preferably, the stentundergoes at least one turn, more preferably at least a plurality ofturns. Repeated turns of the helix along the stent will tend to ensurethat the swirl flow is generated and maintained.

The stent may extend generally linearly. In other words, the axis aboutwhich the centre line of the stent follows a substantially helical path,may be straight. Alternatively the axis may itself be curved, wherebythe envelope occupied by the stented conduit is curved, for example toproduce an “arch” shaped conduit. The bend of the arch may be planar ornon-planar, but should preferably be such that swirl is maintained andnot cancelled by the geometry of the bend. Thus, for example, a stentmay be generally “arch” shaped (planar or non-planar), having thegeometry in accordance with the third aspect of the invention, i.e. suchthat the stented conduit follows a substantially helical path with ahelix angle less than or equal to 65°, and with an amplitude less thanor equal to one half of the internal diameter of the tubing portion.

The stent may if desired comprise a pharmaceutical coating. Such acoating could be provided to provide sustained release of thepharmaceutical over a period of time. So, the stent could provide apharmaceutical for initial treatment of a disease, and in the longerterm the stent gives a therapeutic benefit due to the characteristicswhich it imparts to the flow.

In the above prior art proposals using multiple grooves or ridgesarranged about the tubing circumference, or non-circular sections whichare twisted, where the tubing is substantially straight, then the centreline of the tubing is also straight. This is unlike the centre line ofthe stent of the present invention, in its third aspect, which follows asubstantially helical path. Thus, the stent may have a substantiallycircular cross-section and thus the smallest possible wetted perimeterto cross-sectional area ratio, whilst still having the necessarycharacteristics to induce swirl flow. Of course, there may becircumstances in which the stent has a non-circular cross-section, forexample to assist interfacing or where pressure loss considerations arenot significant.

There are proposals in WO 97/24081 and EP 1127557 A1 for tubing to havea single internal rib arranged helically. This results in the tubinghaving a centre line which follows a helical path, but because the ribis provided in an otherwise cylindrical tube, the amplitude of the helixis very small, generally having a relative amplitude appreciably lessthan 0.05. The generation of swirl flow, if there is any, iscorrespondingly limited and unsatisfactory.

Further concerning the prior art proposals using grooves or ridges orribs, it should be noted that arterial geometry is under normalphysiological conditions non-planar (i.e. curved in more than one planein the nature of a helix) and not grooved or rifled. We have foundexperimentally that at higher relevant Reynolds numbers, the flow in ahelical (non-planar) geometry differs from that in a rifled/groovedgeometry, e.g. there is swirling of both near-wall flow and core flow inthe former case. The development of swirl flow is more rapid than in thecase of rifled/grooved tubing, where swirl flow can take many tubingdiameters to develop. Thus, there is the expectation that theintroduction of the physiological non-planar geometry (unlike grooved orrifled geometry) will be beneficial in respect of inhibiting thedevelopment of pathology.

Because the stent of the third aspect of the invention has a helicalcentre line, there is spatial reorganisation of vortical structures,which results in motion of the core or cores of the axial flow acrossthe section of the stent, promoting mixing across the cross section. Theswirl inhibits the development of stagnation and flow separation regionsand stabilises flows.

As mentioned, in the case of the prior art proposals using multiplegrooves or ridges or ribs, or twisted tubes of a non-circularcross-section, the centre line is straight, not helical. Whilst this canbe expected to stabilise flow at sharp bends, it does not in straighttubes cause spatial reorganisation of vortical structures, resulting inmotion of the core or cores of the axial flow across the section of thetube. Thus it does not promote mixing across the cross section to thesame extent as tubing according to the invention. Such mixing may beimportant in maintaining the mass transport and physiological integrityof the blood vessels.

The stent geometry disclosed herein may be used in various biomedicalapplications e.g. in various arteries (such as in the coronary, carotidand renal arteries), in veins, and in non-cardiovascular applicationssuch as in the gastro-intestinal (e.g. bile or pancreatic ducts),genito-urinary (e.g. ureter or urethra) or the respiratory system (lungairways). Thus, the invention extends to stents for body fluids otherthan blood. In general, the use of the geometry of the invention canavoid the presence of stagnant regions, and hence be beneficial.

Certain preferred embodiments of the invention will now be described byway of example and with reference to the accompanying drawings, inwhich:

FIG. 1 is a perspective view of a first embodiment of stent inaccordance with the invention;

FIG. 2 is a longitudinal cross-sectional of the stent;

FIG. 3 is a transverse cross-sectional view of the stent on the lineIII-III of FIG. 2;

FIG. 4 is a longitudinal cross-sectional view of a second embodiment ofstent;

FIG. 5 is a transverse cross-sectional view of the second embodiment onthe line V-V of FIG. 4;

FIG. 6 is a fragmentary longitudinal cross-sectional view of a thirdembodiment of stent;

FIG. 7 is a fragmentary view of a longitudinal cross-section of a fourthembodiment of stent;

FIG. 8 is a view of an experimental balloon;

FIG. 9 is a view of another experimental balloon;

FIG. 10 is a side view of part of a balloon expandable stent, beforeexpansion;

FIG. 11 is an elevation view of a tubing portion having a flow lumen inaccordance with certain aspects of the invention;

FIG. 12 shows elevation views of tubing portions used in experiments;

FIG. 13 shows elevation views of tubing portions used in furtherexperiments;

FIG. 14 is a perspective view of a stent;

FIG. 15 is a perspective view of another stent;

FIG. 16 is a perspective view of the stent of FIG. 15 internallysupporting an arterial graft part; and

FIG. 17 is a perspective view of an internal arterial stent.

FIGS. 1 to 3 show a woven stent 2 in the expanded condition. The stenthas the usual wire strands 4 arranged in a mesh and collectively forminga mesh like outer wall 7. It is also provided with a helical portion or“stripe” 6 extending longitudinally and circumferentially of the stent.The helical portion 6 in this case consists of two additional strands 8woven into the main mesh.

One effect of the helical portion 6 is to create a cross-sectional shapeapproximating to a circle with a segment removed in the regioncorresponding to the helical portion, as seen in FIG. 3. Thiscross-sectional shape has a centroid 9. The locus of centroids 9 alongthe length of the stent defines a helical centre line 40, shown inFIG. 1. The centre line 40 follows a helical path about a longitudinalaxis 30 which is at the centre of an imaginary cylindrical envelope 20within which the stent is contained. The amplitude A of the helix isshown in FIG. 1.

In practice the amplitude A is greater than would be achieved by merelyrotating the cross-sectional shape about the centre of the circle 3. Inthat case, the envelope 20 would simply correspond to the circle 3.However, the effect of the helical portion resisting extension duringexpansion is to cause the envelope 20 to be appreciably larger than thecircle 3. This is another effect of the helical portion contributing tothe creation of a non-planar or helical flow lumen.

In use, the stent is deployed at a target site and is then expanded by aballoon or by the elasticity or shape-memory properties of the strands4. The helical portion 6 acts to restrict extension (at least in thelongitudinal direction) and hence the expanded stent adopts theconfiguration described, in which the centre line of the stent follows ahelical path. The outer wall 7 engages the fluid conduit wall andinfluences its shape so that the lumen of the fluid conduit at thetarget site also tends to have a helical centre line. This will help topromote swirl flow along the lumen. The handedness (“s” or “z”) of thestent will normally be chosen to complement the local fluid conduitgeometry so as to enhance any swirl flow already existing upstream ofthe stent and not to cancel it.

In the embodiment shown in FIGS. 1 to 3 two helically arranged wires 8are provided, but other numbers of wires could be used. In addition, thewires could be designed to provide the greatest resistance to extensionin the middle of the helical portion 6, with less resistance beingprovided towards the edges of the helical portion, for example bygrading the wires with a thickest wire in the middle and thinner wirestowards the edges. Such an arrangement could ensure that the shape ofthe expanded stent, when viewed in transverse cross-section, does nothave any sharp ridges or grooves and ideally corresponds closely to acircle.

In a modified embodiment a helical portion is formed by a singlehelically arranged wire 8 to produce a stent of substantially circularcross-section. The single wire can provide resistance to longitudinalextension during expansion of the stent and cause it to define a lumenwith a helical centre line. A circular cross-sectional shaped stent canstill provide the desired swirl inducing effect providing the centreline of the lumen is helical.

FIGS. 4 and 5 show an embodiment of a stent of the so-called helicalspiral type. In this case the basic stent design consists of a wire 10in a wave form, shown at 12, with that wave form extending in the mannerof a coil from one end of the stent to the other. Longitudinallyadjacent waves of the wave form 12 are joined by connecting elements 14.In the expanded condition the wavelength of the waves is, for most ofthe circumference of the stent, a distance D. In the region of thehelical portion 6 this wavelength is reduced to less than D. The effectof the reduced wavelength is to cause the lumen of the fluid conduit inwhich the stent is expanded to adopt the desired configuration forpromoting swirl flow in the lumen of the fluid conduit.

In the collapsed condition of the stent of FIGS. 4 and 5 the wavelengthof the waves of the wave form 12 is reduced throughout the stent. Duringexpansion the wavelength in the region of the helical portion 6increases least. Extension in the circumferential direction is resistedby the helical portion 6. This could for example be achieved providingthat the natural shape of the waves in the helical portion 6 is onehaving a smaller wavelength than D. This may be appropriate for exampleif the stent is made by being cut out from a metal sheet or tube.

Another way of achieving the reduced circumferential expansion in theregion of the helical portion 6 would be to provide short bridges 16between circumferentially adjacent portions of the wave in the helicalportion 6. Such a bridge 16 is shown in FIG. 5. Further bridges would beprovided at intervals along the helical portion.

FIG. 7 shows a stent of the closed cell type with “v” hinges betweenadjacent cells. In this case the helical portion 6 is provided byforming a helical line of cells 18 which are smaller than the othercells 20. When the stent is expanded, either by a balloon, or by theelastic or shape-memory properties of the material from which the stentis formed, the cells 20 expand to a predetermined size. The cells 18expand to a smaller predetermined size and hence resist extension. Aswith the other stents, the result is that the lumen of the fluid conduitin which the stent is expanded adopts a configuration promoting swirlflow.

The various stents shown and described are provided with a singlehelical portion 6. However, other numbers of helical portions could beprovided. Preferably the stents are non-rotationally symmetrical(rotational symmetry of order 1), as this can ensure that the centreline of the expanded stent follows a helical path.

FIG. 14 shows an internal stent 12 for use in a graft vessel or anintact vessel. The stent 12 is fabricated from a linked wire mesh andhas a helical form along substantially the whole length of the stent.The material used is preferably a shape memory alloy to facilitateinsertion of the stent.

FIG. 15 shows an alternative embodiment of an internal stent 12, inwhich the linked wire mesh has a helical tubing portion 1 only over ashort region at one end thereof.

FIG. 16 shows the stent 12 located in a graft 14 post insertion. Thegraft 14 surgically attached to an artery 6 has been shown transparentfor purposes of illustration, to show the internally located, parthelical wire mesh stent in-situ.

In the case of the internal stents of FIGS. 14, 15 and 16, in order toavoid the mesh itself forming a honeycomb of stagnant regions, amodification may involve providing the mesh with a smooth internallining. Alternatively, an inner layer of the stent may be helicallywound, without linkages, as described in WO 01/45593.

The helical form of the stents is arranged to promote swirl flow andthereby minimise flow instability and development of pathology.

The stents shown in FIGS. 14 and 15 are defined within an envelope witha curved longitudinal axis. They are generally arch shaped. Such an archmay curve in a single plane or may itself be non-planar, in which casethe non-planarity should promote swirl flow in the same direction as thehelix.

The stents need not be arch shaped; they may instead have a generallystraight axis, as shown for example in FIG. 17. The stent of FIG. 17 hasa straight central longitudinal axis 30, with a helical centre line 40which undergoes about half of one turn. The low amplitude of the helixof the stent means that it is close to a cylindrical shape and cantherefore be used in procedures where conventional stents wouldpreviously have been used. However, this is achieved without departingfrom a circular cross-sectional shape and without using helical ribs orother projecting formations. It is expected that the vessel into whichthe stent is inserted will be able to adopt the shape defined by thestent and therefore benefit from swirl flow. In other embodiments, thehelical centre line may undergo more than half of one turn, and indeedmore than one or more turns.

The stent of FIG. 17 may be useful as an arterial stent where there isthrombosis or stenosis of for example coronary arteries.

FIG. 8 shows the result of an experiment carried out on a toy balloon55. The balloon was of the elongated type. It was supported, withoutbeing inflated, on a cylindrical rod and a plastic strip 51 cut fromanother balloon was glued onto the outside of the supported balloon toform a longitudinally and circumferentially extending helical strip 6. Astraight line 50 was drawn along the balloon. After the glue had set,the balloon was inflated and the inflated balloon is shown in FIG. 8.

It will be seen that the inflated balloon 55 has a helical lumen. Aswith the stents, it has a helical centre line 40, which follows ahelical path about a longitudinal axis 30. The longitudinal axis is atthe centre of an imaginary cylindrical envelope 20 within which theballoon is contained. The amplitude A of the helix is shown in FIG. 8.

It will be noted that after inflation the straight line 50 adopts a waveshape which remains consistently along the same side of the balloon, sothat the entire line 50 remains visible in the elevation view of FIG. 8.This indicates that the balloon has gone from the collapsed condition tothe inflated condition without any significant twisting. There is no nettwisting along the length of the balloon. A similar effect in anexpanding stent in accordance with the preferred embodiments of theinvention means that as the stent expands and engages the inside of afluid conduit in which it is sited it does not impose excessivetorsional loads on that conduit. This is beneficial in the case of theconduit being a blood vessel, for example, since torsion is resisted bythe external tethering of the blood vessel.

Thus in the preferred embodiments the stents expand from the collapsedcondition to the expanded condition without substantial twisting. Thelack of twisting of the stent also enables it to have a plurality ofturns without causing e.g. a blood vessel to twist during expansion,such twisting being undesirable because of the tethering of the bloodvessel.

The balloon of FIG. 8 starts as a cylindrical membrane with a helicalportion which is of greater (in this case double) wall thickness thanthe rest of the balloon. During inflation the thicker helical portionwill tend to resist extension in all directions, includingcircumferential and longitudinal directions, thereby influencing theshape of the expanded balloon. Instead of adopting the normalcylindrical shape, the balloon forms a shape with a helical centre line40.

FIG. 8 shows that the amplitude A achieved by the helical portion ismuch greater than would be achieved by simple rotation of a non-circularcross-section. The diameter of the envelope 20 is substantially greaterthan the diameter of the balloon. The same effect is obtained for anexpanded stent outer wall or the wall of a balloon used in a balloonexpandable stent.

In another experiment, a plastic strip 52 was made with a tapered width,rather than with parallel side edges. It was found that the amplitude Aof the helical centre line 40 was larger where the width of the stripwas wider. This is shown in FIG. 9. A thinner strip tends to cause lessdeviation of the cross-sectional shape of the balloon from a circularshape.

The shape of the expanded experimental balloon membranes may beconsidered as analogous to that of an expanded stent outer wall or thewall of a balloon used in a balloon expandable stent. Consideringtherefore the inside of the helical balloon as a lumen or flow path, itwill be appreciated that a helical lumen is obtained, giving thedesirable flow properties discussed above, without the use of ribs,vanes or other flow guides protruding into the flow.

The experimental results thus show that by introducing a helical portionwhich resists extension during expansion of a stent, when expanded thestent will adopt a shape causing a fluid conduit which it supports tohave a helical lumen, thereby promoting swirl flow. The effect observedin the balloons of FIGS. 8 and 9 may be obtained in a main stent body,either self-expanding or balloon expandable, and/or in a balloon whichis used to expand a balloon expandable stent.

FIG. 10 shows a balloon expandable stent 2. A balloon 55 is providedwith a helical strip 6. Upon inflation, the balloon causes the stent 2to adopt the desired helical geometry, expanding to a shape as shown inthe experimental balloon of FIG. 8. The stent is designed to deformplastically so that it holds to the shape supporting a conduit e.g.blood vessel so as to have a helical flow lumen.

It will be noted that in the preferred embodiments the stent does notrely on the use of thicker wires which themselves provide ribs or flowguides in an otherwise circular cross-section lumen. Rather, the shapeof the lumen is modified by the resistance of the helical portion toextension.

The tubing portion 1 shown in FIG. 11 has a circular cross-section, anexternal diameter D_(E), an internal diameter D_(I) and a wall thicknessT. The tubing is coiled into a helix of constant amplitude A (asmeasured from mean to extreme), constant pitch P, constant helix angle θand a swept width W. The tubing portion 1 is contained in an imaginaryenvelope 20 which extends longitudinally and has a width equal to theswept width W of the helix. The envelope 20 may be regarded as having acentral longitudinal axis 30, which may also be referred to as an axisof helical rotation. The illustrated tubing portion 1 has a straightaxis 30, but it will be appreciated that in alternative designs thecentral axis may be curved. The tubing portion has a centre line 40which follows a helical path about the central longitudinal axis 30.

It will be seen that the amplitude A is less than the tubing internaldiameter D_(I). By keeping the amplitude below this size, the spaceoccupied by the tubing portion can be kept relatively small, whilst atthe same time the helical configuration of the tubing portion promotesswirl flow of fluid along the tubing portion.

EXAMPLE 1

Experiments were carried out using polyvinyl chloride tubing with acircular cross-section. Referring to the parameters shown in FIG. 12 thetubing had an external diameter D_(E) of 12 mm, an internal diameterD_(I) of 8 mm and a wall thickness T of 2 mm. The tubing was coiled intoa helix with a pitch P of 45 mm and a helix angle θ of 8°. The amplitudeA was established by resting the tubing between two straight edges andmeasuring the space between the straight edges. The amplitude wasdetermined by subtracting the external diameter D_(E) from the sweptwidth W:2A=W−D _(E)So:

$A = \frac{W - D_{E}}{2}$

In this example the swept width W was 14 mm, so:

$A = {\frac{W - D_{E}}{2} = {\frac{14 - 12}{2} = {1\mspace{11mu}{mm}}}}$

As discussed earlier, “relative amplitude” A_(R) is defined as:

$A_{R} = \frac{A}{D_{I}}$

In the case of this Example, therefore:

$A_{R} = {\frac{A}{D_{I}} = {\frac{1}{8} = 0.125}}$

Water was passed along the tube. In order to observe the flowcharacteristics, two needles 80 and 82 passing radially through the tubewall were used to inject visible dye into the flow. The injection siteswere near to the central axis 30, i.e. at the “core” of the flow. Oneneedle 80 injected red ink and the other needle 82 blue ink.

FIG. 11 shows the results of three experiments, at Reynolds numbersR_(E) of 500, 250 and 100 respectively. It will be seen in all casesthat the ink filaments 84 and 86 intertwine, indicating that in the corethere is swirl flow, i.e. flow which is generally rotating.

EXAMPLE 2

The parameters for this Example were the same as in Example 1, exceptthat the needles 80 and 82 were arranged to release the ink filaments 84and 86 near to the wall of the tubing. FIG. 13 shows the results of twoexperiments with near-wall ink release, with Reynolds numbers R_(E) of500 and 250 respectively. It will be seen that in both cases the inkfilaments follow the helical tubing geometry, indicating near-wallswirl. Furthermore, mixing of the ink filaments with the water ispromoted.

It will be appreciated that this invention, in its third and fourthaspects, is concerned with values of relative amplitude A_(R) less thanor equal to 0.5, i.e. small relative amplitudes. In a straight tubingportion both the amplitude A and the relative amplitude A_(R) equalzero, as there is no helix. Therefore, with values of relative amplitudeA_(R) approaching zero, the ability of the tubing portion to induceswirl will reduce. The lowest workable value of relative amplitude A_(R)for any given situation will depend on the speed of flow and theviscosity and density of the fluid (i.e. Reynolds number) and on thepitch (helix angle) and the particular use of the tubing portion.Relative amplitudes of at least 0.05, 0.10, 0.15, 0.20, 0.25, 0.30,0.35, 0.40 or 0.45 may be preferred.

The invention claimed is:
 1. An expandable balloon for insertion in afluid conduit of the human or animal body, in combination with a stent,the balloon being movable between a collapsed condition and an expandedcondition, and the balloon being supported in the collapsed condition ona shaft, and the balloon having, when in the expanded condition, acentre line which follows a substantially helical path whereby theballoon has a helical shape wherein the helical centre line of theballoon has a helix angle less than or equal to 65°, wherein the balloonhas a substantially circular cross-section without helical ribs orhelical vanes protruding into an interior of the balloon, wherein thestent is expandable by the balloon from a collapsed condition to anexpanded condition, and wherein the expanded condition the stent has ahelical shape which corresponds to the helical shape of the balloonincluding the substantially helical centre line thereof such that theexpanded stent has said substantially helical centre line.
 2. Theballoon as claimed in claim 1 wherein, after expansion of the balloon,the stent retains, by plastic deformation, a shape which corresponds tothat determined by the balloon.
 3. The balloon as claimed in claim 1,wherein in the expanded condition of the balloon, wherein an amplitudeof the helical centre line of the balloon is less than or equal to onehalf of the internal diameter of the substantially circularcross-section of the balloon.
 4. The balloon as claimed in claim 1,wherein the balloon comprises an expandable wall and the wall has ahelical portion which extends longitudinally and circumferentially, andwherein the helical portion, upon expansion of the balloon from thecollapsed condition to the expanded condition, resists extension.
 5. Theballoon as claimed in claim 4, wherein the helical portion of theexpandable wall has a greater thickness than that of adjacent wallportions.
 6. The balloon as claimed in claim 1, wherein the helicalcentre line of the balloon has a helix angle less than or equal to 35°.7. An expandable balloon for insertion in a fluid conduit of the humanor animal body, the balloon being movable between a collapsed conditionand an expanded condition, the balloon having, when in the expandedcondition, a centre line which follows a substantially helical pathwhereby the balloon has a helical shape wherein the helical centre lineof the balloon has an amplitude and has a helix angle less than or equalto 65°, wherein the balloon is configured to impose its helical shapeincluding the substantially helical centre line thereof on a vessel inwhich it is to be inserted, whereby the vessel is caused to have saidsubstantially helical centre line, wherein the balloon has asubstantially circular cross-section without helical ribs or helicalvanes protruding into an interior of the balloon, and wherein in theexpanded condition of the balloon, the amplitude of the helical centreline of the balloon is less than or equal to 0.5 of the internaldiameter of the substantially circular cross-section of the balloon.